This application is a U.S. national counterpart application of international application serial No. PCT/AU00/00388 filed May 1, 2000, which claims priority to Australian application serial no. PQ 0071 filed May 3, 1999.
This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals which have high solidification temperatures and tend to produce defects caused by uneven solidification at the chilled casting surfaces of the rolls. One particular problem arises due to the formation of pieces of solid metal known as xe2x80x9cskullsxe2x80x9d in the vicinity of the pool confining side plates. These problems are exacerbated when efforts are made to reduce the superheat of the incoming molten metal. The rate of heat loss from the melt pool is greatest near the side plates due primarily to additional conductive heat transfer through the side plates to the roll ends. This high rate of local heat loss is reflected in the tendency to form xe2x80x9cskullsxe2x80x9d of solid metal in this region which can grow to a considerable size and fall between the rolls causing defects in the strip generally known as xe2x80x9csnake eggsxe2x80x9d. It is therefore very important to maintain constant pool conditions in the region of the side plates. In particular, the setting of the gaps between the nozzle ends and the inner faces of the side plates is critically important.
We have determined that significant flow changes are brought about by variation in the position in the ends of the delivery nozzle relative to the side plates which may be brought about by inaccurate location of the delivery nozzle during set up and by subsequent movement of the nozzle ends due to thermal expansion during casting. This problem remains even if the nozzle is designed specifically to provide an increased flow of metal to the xe2x80x9ctriple pointxe2x80x9d regions (ie. where the side dams and casting rolls meet in the meniscus regions of the casting pool) to increase the heat input to these regions of the pool. Examples of such nozzles may be seen in U.S. Pat. Nos. 4,694,887, 5,221,511 and our earlier Australian Patent Application 35218/97 based on Provisional Application PO2367.
Although triple point pouring has been effective to reduce the formation of skulls in the triple point regions of the pool it has not been possible completely to eliminate the problem because the generation of defects is remarkably sensitive to even minor variations in the flow of metal into the triple point regions of the pool and movements of the nozzle ends due to thermal expansion during casting can be sufficient to cause defects. As the gap between the nozzle end and the side plate is reduced the downwardly inclined flow of metal from the triple point pouring passages in the ends of the nozzle impinges higher on the side plates. This can lead to the formation of skulls with subsequent snake egg defects or in extreme cases can cause the poured metal to surge upwardly in the reduced gap between the nozzle ends and side plates to spill over the upper edges of the side plates. This problem is addressed by the invention disclosed in our Australian Patent Application 63175/99 which provides an improvement by which it is possible to maintain substantially constant spacing between the nozzle ends and the side plates throughout casting.
According to the invention disclosed in Application 63175/99 there is provided apparatus for casting metal strip including
a pair of parallel casting rolls forming a nip between them,
an elongate metal delivery nozzle formed in a plurality of discrete elongate pieces disposed end to end,
nozzle support means supporting the nozzle pieces such that the nozzle extends above and along the nip between the casting rolls for delivery of molten metal into the nip whereby to form a casting pool of molten metal supported above the nip,
a pair of pool confinement plates at the ends of the nip,
plate biasing means to bias the pool confinement plates against end surfaces of the rolls so that the plates move inwardly of the rolls to accommodate wear of the plates, and
nozzle end shifter means to shift the nozzle pieces defining the outer nozzle ends on the support means with inward movements matching the inward movements of said side plates accommodating wear of the side plates thereby to maintain substantially constant spacings between the side plates and the nozzle ends.
In the specific apparatus disclosed in Application PP8024 the nozzle end shifter means comprised spacers disposed between the nozzle ends and the side plates to set the spacings between the nozzle ends and the side plates and through which the side plates push the nozzle ends inwardly as they move inwardly under the influence of the biasing means to accommodate wear of the side plates. We have now developed an alternative means for shifting the nozzle ends to provide more reliable control of the spacing between the nozzle ends and the side plates throughout casting. In accordance with this development the invention includes apparatus for casting metal strip including
a pair of parallel casting rolls forming a nip between them,
an elongate metal delivery nozzle formed in a plurality of discrete elongate pieces disposed end to end,
nozzle support means supporting the nozzle pieces such that the nozzle extends above and along the nip between the casting rolls for delivery of molten metal into the nip whereby to form a casting pool of molten metal supported above the nip,
a pair of pool confinement plates at the ends of the nip,
plate biasing means to bias the pool confinement plates against end surfaces of the rolls so that the plates move inwardly of the rolls to accommodate wear of the plates, and
nozzle end shifter means to shift the nozzle pieces defining the outer nozzle ends on the support means with inward movements matching the inward movements of said side plates accommodating wear of the side plates thereby to maintain substantially constant spacings between the side plates and the nozzle ends, wherein the nozzle end shifter means comprises a pair of moveable structures disposed one at each end of the casting roll assembly, moving means to move those structures longitudinally of the rolls, nozzle attachment means attaching the moveable structures to the two nozzle end pieces defining the outer nozzle ends so that those two nozzle pieces are moved with the movable structures, and control means responsive to inward advances of the side plates relative to the outer ends of the rolls to cause the moving means to move the moveable structures inwardly and so shift said two nozzle pieces with matching inward movements.
The plate biasing means may comprise a pair of generally horizontally acting thrusters actuable to apply opposing inward closure forces to the pool confinement plates and said moveable structures may provide abutments against which the thrusters react to apply the inward closure forces to the plates.
The moveable structures may comprise a pair of carriages which carry the thrusters and which are moveable toward and away from another to enable the spacing between them to be adjusted so that the carriages can be preset before a casting operation to suit the width of the casting rolls.
The moveable structures may include carriage drive means acting between outer end parts of the moveable structures and the carriages to move the carriages toward and away from one another.
The carriage drive means may comprise a pair of fluid operable cylinder units connected one to each of the carriages and to outer end parts of said moveable structures.
The moving means may act on the outer end parts of the moveable structures.
The moving means may comprise a pair of jacks connected to the outer end parts of the moveable structures. Those jacks may be electrically driven screw operated jacks.
The control means may be responsive to motion of the plate biasing means which produces inward movements of the pool confinement plates. The control means may, for example, include transducers in the plate thrusters to produce control signals indicative of movement of the thrusters and plates and connected in a control circuit with the moving means such that the moving means causes corresponding movements of the moveable structures and therefore said two nozzle pieces.
Alternatively the control means may include inspection means to observe the position of the pool confinement plates and to provide control signals dependant on observed changes in the position of those plates.
The moving means may be independently operable to adjust the initial setting of said two nozzle pieces relative to the plates.